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Diode and zener-diode as voltage divider?
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voltampere:
Probably easy for the experts here, but i am scratching my head too long now about this topic.

I am troubleshooting the power supply of a Fluke 5440A and have a burnt zener diode.
Before i just replace the zener with a spare i want to fully understand the function of this part of the circuit.

Here is the part of the schematic:



I have drawn a red line to show the relevant path.
As i see it there is no series resistance for the diode and zener combination.
So the only resistane would be the combined impedance of the zener and diode.
At 22 volts across them how can you keep these parts from glowing?

From the "List of replacable Parts":

CR  5  DIODE, SI, BV= 75.0V, IO= 150MA, 500MW   1N4448
VR 13 ZENER, UNCOMP, 5.1V, 5%, 20.0MA, 0.4W     1N751A

Thanks for helping

Joerg
Gyro:
It's most likely to prevent the zener conducting in the reverse direction. Remember that if you reverse the polarity across a zener it becomes an ordinary diode. If, say, it's between two supply rails, if one gets shorted you don't want the other one to get dragged down too. The series diode ensures that you only get conduction in the 'zener' direction.
voltampere:
Makes sense...

But to me the question still remains:

How can the zener ( and the diode ) survive without some current limiting resistance?

So far every explanatory circuit with a zener that i could find had a resistor in series.

regards

joerg
Psi:
Maybe the L4 inductor has enough resistance to keep the diode current within spec.

A small 5x5x2mm SMT inductor can have 2k of resistance
etc SRR4528A-391MDKR-ND

Also i dunno where the coil is for that relay, maybe if current flows through the diodes it pulls the relay in and breaks current through the diode? ie, self protecting.
voltampere:
Okay, i did not expect the inductor to have much impact.
2k ohm would definitely help here.
But your datasheet reference gives 1910 typ. and 2380 max, but that is milliohms!

The incuctor is described as "6 turn choke".

I checked the datasheet of a 1N751A and it says max regulator current (IZM) 70 mA.
So if i calculate it then the whole "string" should have not less than 315 ohm.

Is this possible?
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